In an overfilled symmetrical laser printer (OSLP) it is necessary to compensate for the fall off in exposure from the center to the edge of the scan caused by the deflector surface scanning across the Gaussian intensity shape of the input beam. This is a fall off in one dimension, namely the line scan direction (the beam is focused in the page direction so the deflector is not overfilled in the page direction). It is desirable to correct this shape by reshaping the beam intensity profile to be flatter, or even smile shaped to compensate other sources of exposure fall off. It is desirable to have a one dimensional correction that minimizes the loss of light.
U.S. Pat. No. 4,941,721 discloses the use of two aspehric lenses to achieve the desired beam profile for use in an input scanner rather than output printer. However, it is not a symmetrical system and requires the fabrication of aspheres. U.S. Pat. No. 3,465,347 discloses an absorbing filter but it is a thick absorber, glass or liquid, two dimensional, and used in a nonsymmetrical scanner. Articles by Han, Roberts, Veldkamp, Roux, and Aleksoff describe computer fabricated diffractive optics schemes which can be low in transmission and are wavelength sensitive, Han, Ishii and Murata, Reshaping Collimated Laser Beams with Gaussian Profile to Uniform Profiles, Applied Optics, Vol. 22, No. 22, (1983), N. C. Roberts, Beam Shaping by Holographic Filters, Applied Optics, Vol. 28, No. 1, (1989), W. B. Veldkamp, Laser Beam Profile Shaping with Interlaced Binary Diffraction Gratings, Applied Optics, Vol. 21, No. 17, (1982), F. S. Roux, Intensity Distribution Transformation for Rotationally Symmetric Beam Shaping, Optical Engineering, Vol. 30, No. 5, (1991) and Aleksoff, Ellis and Neagle, Holographic Conversion of a Gaussian Beam to a Near-Field Uniform Beam, Optical Engineering, Vol. 30, No. 5 (1991).
U.S. Pat. No. 5,061,046 discloses a gradient index liquid crystal device. A laser beam apodizer using cholesteric liquid crystals provides a soft edge profile using two separate cholesteric liquid crystal mixtures with different selective reflection bands, which, in an overlap region, have a gradient index where reflectivity changes as a function of position. The apodizer can be configured as a one dimensional beam apodizer with a clear aperture which can be mechanically adjusted by sliding two complimentary devices relative to each other, and as a circular beam apodizer.
Dew and Parsons, Absorbing Filter to Flatten Gaussian Beams, Applied Optics, Vol. 31 No. 18, Jun. 20, 1992, disclose an absorbing filter that flattens the Gaussian intensity profile of a laser beam. The filter was fabricated by dc magnetron sputtering a thin tantalum film, by using a combination of substrate masking and motion.
It will be appreciated that it would be highly desirable to compensate for Gaussian intensity fall off without the use of complicated, bulky or expensive components, such as aspheres or liquid crystals, and without using complicated or expensive methods, such as dc magnetron sputtering. A simple apodizer can be made on photographic film or plates, and such apodizers are typically fabricated by contact projection or variable exposure printing. It is desirable to fabricate an apodizer on a photographic film or plate that is inverse Gaussian in transmission and simple to fabricate.